Fluorinated carbons or graphite fluorides have a chemical formula CFx, where, typically, O<x<1.14. Most techniques for the production of fluorinated carbons employ direct fluorination of graphites or petroleum cokes at 600-900 k, with properties dependent upon the structure and antecedents of the starting material that is used. Depending upon the fluorine content, CFx exhibits a wide range of electrical resistivities, has a low coefficient of friction, a low surface free energy and a range of colour from black (x<0.8) to white (x>0.95). Graphite fluorides consist of layers of (CF)n puckered in an infinite array of cyclohexane chairs.
The excellent lubricity and water and oil repellency properties of graphite fluorides have been utilised in various composite materials such as plastics, organic and aqueous liquids, and also as cathodic materials in high energy density lithium batteries. For such applications, the dispersability of graphite fluoride is an important factor; since dispersability is primarily dependent upon particle size, there is a requirement for nano-sized particles of fluorinated carbon.
An alternative approach to the production of fluorinated carbons, although one which has been much less extensively used, is to fluorinate carbon black. Carbon black is a well known substance formed of spheroidally-shaped particles grouped together into chains or clusters known as aggregates. Carbon black is formed by the dissociation of hydrocarbons and finds use as a filler for rubber products, in the manufacture of printing inks, tinting, and in paper and fibre colourings. Traditional methods for the production of carbon black (such as lamp black, furnace black and gas black) relied upon partial combustion of petrochemical and coal tar oils. Over recent decades, however, plasma systems have also been employed as they are typically more controllable, efficient and environmentally friendly. Moreover, unique properties and characteristics are seen in carbon blacks generated by a plasma process. U.S. Pat. Nos. 5,527,518, 5,989,512, 5,997,837 and 6,068,827 describe examples of the generation of carbon blacks with a plasma torch, using a quenching technique which allows the size of carbon black particles to be controlled and permits a 100% yield to be obtained.
Production of fluorinated carbons on an industrial scale is difficult. The conditions must be strictly controlled and for direct fluorination, long reaction times are required in order to completely fluorinate a carbon material. This results in the use of large quantities of expensive fluorine. The yield is in any event low and large quantities of gaseous by-product are formed. Plasma fluorination techniques have been employed on a limited scale, but require expensive equipment, that is time-consuming to operate. The requirements for a vacuum in the plasma generation techniques also limit such processes to small batches which in turn makes industrial production difficult.
It is an object of the present invention to provide an improved process for the production of fluorinated carbons and in particular for the production of fluorinated carbon black.
According to the present invention, there is provided a process for producing a fluorinated carbon material comprising the steps of generating a plasma in a plasma chamber; and supplying a fluorocarbon or fluorocarbon containing mixture to the plasma whereby at least some of the fluorocarbon transforms to a fluorinated carbon material.
Direct pyrolysis of a fluorocarbon or fluorocarbon containing mixture provides a ‘one-step’ process which avoids the problems of the two-step prior art processes, particularly in that the overall production time is reduced and that large amounts of fluorine gas are not now needed. A perfluorinated or partially fluorinated fluorocarbon is typically employed as a feedstock with a nitrogen or argon plasma gas supplied to form an atmospheric pressure plasma. By the term “fluorocarbon” is meant any of a number of organic compounds analogous to hydrocarbons, where either some or all of the hydrogen atoms have been replaced by fluorine. In preference, therefore, hydrofluorocarbons having both hydrogen and fluorine atoms as well as carbon may be used as a feedstock.
The resultant product has the form (CFx), where x is ≦0.8, preferably from 0.05 and 0.30, and more preferably from 0.06 to 0.15. The fluorinated carbon is similar in appearance to carbon black; the particles have a relatively narrow spread of diameters (the mean particle diameter is in the range of from 10 to 100 nm, preferably 15 to 60 nm, and more preferably from 15 to 50 nm, and at least 90% and preferably substantially all of the particles have a diameter falling within the range of from 15 to 50 nm). The particles also exhibit excellent hydrophobicity when used as a substrate coating.
In a preferred embodiment, the fluorinated nanocarbon material may be doped with other elements such as chlorine, oxygen or nitrogen. The invention accordingly extends, in a preferred embodiment, to a fluorinated carbon material thus doped.
The fluorinated carbon produced in accordance with the invention finds a wide range of applications including, but not limited to, electrodes for high energy lithium cells, an additive for a photocopy toner, an ink, a super-hydrophobic coating or a filler for a fluoropolymer.